A Global Navigation Satellite System (GNSS) includes a network of satellites that broadcast radio signals, enabling a user to determine the location of a receiving antenna with a high degree of accuracy. Examples of GNSS systems include Navstar Global Positioning System (GPS), established by the United States; Globalnaya Navigatsionnay Sputnikovaya Sistema, or Global Orbiting Navigation Satellite System (GLONASS), established by the Russian Federation and similar in concept to GPS; and Galileo, also similar to GPS but created by the European Community and slated for full operational capacity in 2008.
Currently the best-known of the available GNSS, GPS was developed by the United States government and has a constellation of 24 satellites in 6 orbital planes at an altitude of approximately 26,500 km. Each satellite continuously transmits microwave L-band radio signals in two frequency bands, L1 (1575.42 MHz) and L2 (1227.6 MHz). The L1 and L2 signals are phase shifted, or modulated, by one or more binary codes. These binary codes provide timing patterns relative to the satellite's onboard precision clock (synchronized to other satellites and to a ground reference through a ground-based control segment), in addition to a navigation message giving the precise orbital position of each satellite, clock correction information, and other system parameters.
The binary codes providing the timing information are called the C/A Code, or coarse acquisition code, and the P-code, or precise code. The C/A Code is a 1 MHz Pseudo Random Noise (PRN) code modulating the phase of the L1 signal and repeating every 1023 bits (one millisecond). The P-Code is also a PRN code, but modulates the phase of both the L1 and L2 signals and is a 10 MHz code repeating every seven days. These PRN codes are known patterns that can be compared to internal versions in the receiver. The GNSS receiver is able to compute an unambiguous range to each satellite by determining the time-shift necessary to align the internal code to the broadcast code. Since both the C/A Code and the P-Code have a relatively long “wavelength”—approximately 300 meters (or 1 microsecond) for the C/A Code and 30 meters (or 1/10 microsecond) for the P-Code, positions computed using them have a relatively coarse level of resolution.
Commonly it is desirable to improve the accuracy, reliability, or confidence level of an attitude or position determined through use of a GNSS, a Satellite-Based Augmentation System (SBAS) may be incorporated if one that is suitable is available. There are several public SBAS that work with GPS. These include Wide Area Augmentation System (WAAS), developed by the United States' Federal Aviation Authority, European Geostationary Navigation Overlay Service (EGNOS), developed by the European Community, as well as other public and private pay-for-service systems such as OmniSTAR®.
Conventional GPS antennas include ceramic patch, cross diploes and microstrip patch. The ceramic patch is of compact size and has the benefit of low cost but its bandwidth is narrow and it cannot be used in high accuracy applications. The cross dipole antenna has a high gain at low elevation angles and consequently exhibits less desirable multipath performance. It also has complicated assembly issues. There are numerous microstrip patch antennas in the art including commonly assigned U.S. Pat. No. 5,200,756 issued to Feller. This three dimensional microstrip patch antenna has high gain at low elevation angles but it exhibits less desirable multipath performance. U.S. Pat. No. 6,252,553, issued to Solomon is a multi-mode patch antenna system and method of forming and steering a spatial null. This antenna uses four feed probes and geometrical non-symmetry and the radiating patch is assembled over the ground plane. The active circuit employed also requires an additional circuit card. U.S. Pat. No. 6,445,354, issued to Kunysz is termed a pinwheel antenna design. The pinwheel antenna has nice performance including the ability to reduce multipath but it is difficult to manufacture compared to other antenna configurations. This antenna also employs two circuit cards, an RF absorber and a cable connection between both cards. U.S. Pat. No. 6,597,316 issued to Rao, et al., is a spatial null steering microstrip antenna array. This antenna also exhibits good multipath reducing properties and accuracy but its feed circuit is comparatively complicated, consisting of four coaxial probes and three combiners.
With respect to the existing designs for antennas, there still remains a need for improvements in compact packaging, ease of assembly, broadband reception, multipath mitigation, accuracy and sensitivity. It is also desirable to realize the above improvements using a microstrip patch antenna design. It is further desirable to provide a GPS antenna with broadband capabilities that covers both GPS signal bands and L-Band signals such as those broadcast for augmentation and differential corrections such as OmniSTAR® and the like. It would also be desirable to combine radiator, coupling apertures, feed circuit and active circuit into one single circuit card to enhance and compact structure, facilitate assembly, and ensure lower cost.